docs: def declaration_files(decl_or_decls):
      """
      Returns set of files
  
      Every declaration is declared in some file. This function returns set, that
      contains all file names of declarations.
  
      :param decl_or_decls: reference to list of declaration's or single
          declaration
      :type decl_or_decls: :class:`declaration_t` or [:class:`declaration_t`]
      :rtype: set(declaration file names)
  
      """
  
      files = set()
      decls = make_flatten(decl_or_decls)
      for decl in decls:
          if decl.location:
              files.add(decl.location.file_name)
      return files